This invention relates to a data communication system for transmitting voice signal and data signal simultaneously using a communication line for voice signal such as a telephone line.
Methods for transmitting voice signal and data signal simultaneously using a telephone line such as a method disclosed in Japanese Patent Provisional Publication Heisei 3-110988 (1991) and a method standardized as a standard of tele-writing which is a system for transmitting data signal such as hand written graphic during talking have been proposed. In these methods, a part of voice band is utilized as band for data signal, and data signal is modulated to multiplex with voice signal.
However, the above mentioned technologies are disadvantageous in that when voice signal and data signal are transmitted simultaneously in the situation that the type of the system of a counterpart is unknown. Further when the signal is received with a conventional telephone, voice mixed with a modulated data signal is received, and talking is impossible and uncomfortable for the receiver, due to employing frequency a multiplexing system which multiplexes data signal on a part of voice band. The above-described calling causes no problem if the of transmitter and receiver are of the same type of system.